Trusses work on a bridge by efficiently distributing the weight of the bridge and traffic loads through a network of interconnected members that primarily experience forces of tension or compression.
At its core, a truss is a series of individual members, acting in tension or compression and performing together as a unit. This unique structure allows the bridge to support significant weight while often using less material than other bridge designs like beam or arch bridges.
Understanding the Forces: Tension and Compression
The effectiveness of a truss bridge lies in how its members handle two fundamental forces:
- Compression: This is a pushing force that shortens or compresses a member. Imagine pushing inward on the ends of a stick – that's compression.
- Tension: This is a pulling force that stretches or lengthens a member. According to the reference, a tension member is subject to forces that pull outward at its ends. Think of pulling a rope taut – that's tension.
Within a truss structure, some members will be under compression, while others will be under tension, depending on their position and the load applied.
How Members Work Together as a Unit
When a load is placed on a truss bridge, the force is transferred through the interconnected members. Because the members are arranged in rigid, often triangular, patterns, they work collectively to spread the load across the entire structure and ultimately direct it down to the bridge's supports (abutments or piers).
- Load Distribution: Instead of a single beam bending under weight, the load is broken down into forces of tension and compression within the truss members.
- Efficiency: This distribution means that individual members only need to be strong enough to handle these specific axial (along the length of the member) forces, rather than complex bending forces. This often allows for lighter, more economical structures.
Components of a Truss Bridge
Even on older bridges, the individual components are crucial. As the reference notes, even on a "wooden" truss bridge, these members are often individual metal pieces such as bars or rods. This highlights that regardless of the primary material, the concept of discrete members handling specific forces is key.
Here's a simple breakdown of how forces might affect members:
| Member Type | Primary Force Direction | Effect on Member | Example in Truss (Conceptual) |
|---|---|---|---|
| Compression Member | Pushed inward at ends | Shortens | Top cords, vertical posts |
| Tension Member | Pulled outward at its ends | Stretches/Lengthens | Bottom cords, diagonal braces |
By arranging these tension and compression members strategically, the truss transforms vertical downward forces (like gravity and traffic load) into horizontal and angled forces that are safely managed by the structure. This cooperative action of individual members acting as a unit is the fundamental principle behind how trusses work on a bridge to provide strength and stability.
